The dolphin biosonar beam is focused in stages: validation of a vibroacoustic finite element model using bottlenose dolphin simulations

Environmental concern about the potential impact of anthropogenic sounds on aquatic life has sparked increased interest in marine bioacoustics. Experiments with live organisms are difficult to conduct and require considerable resources. Computerize numerical modelling is economical, reduces the need to expose live animals, and increases our understanding of bioacoustic interactions. Computer models should always be validated by comparing their simulations against results gleaned from live organisms. We investigated toothed whale biosonar by developing a numerical model that simulates the vibroacoustic functions of the biosonar apparatus (Krysl et al. 2008). In order to validate this approach, we used a vibroacoustic finite element model to recreate sound production and acoustic beam formation in the bottlenose dolphin (Tursiops truncatus). The model is constructed from live and post-mortem dolphin CT scans, tissue property measurements, and custom software. The right and left dorsal bursae were assumed to be the sound sources (Cranford 1988, 1992). This model confirms several hypotheses from previous studies: (1) the shape of the skull plays a role in the formation of the sound beam; (2) the melon has a significant capacity to focus the transmitted beam; (3) focusing the sound beam apparently happens in a series of stages that include contributions from the skull, nasal diverticula, melon, and connective tissue structures. An unexpected result is that adjustments to the focus and direction of the sound beam can result from small (millimetre scale) changes in the relative position of the anterior and posterior bursa within each sound generation complex. A comparison of our results with those from live dolphin psychoacoustic experiments (Au et al. 1986) supports validation of our vibroacoustic model.